Declaration

Overview

An operation queue executes its queued Operation objects based on their priority and readiness. After being added to an operation queue, an operation remains in its queue until it reports that it is finished with its task. You can’t directly remove an operation from a queue after it has been added.

Note

Operation queues retain operations until they're finished, and queues themselves are retained until all operations are finished. Suspending an operation queue with operations that aren't finished can result in a memory leak.

Determining Execution Order

Operations within a queue are organized according to their readiness, priority level, and interoperation dependencies, and are executed accordingly. If all of the queued operations have the same queuePriority and are ready to execute when they are put in the queue—that is, their isReady property returns true—they’re executed in the order in which they were submitted to the queue. Otherwise, the operation queue always executes the one with the highest priority relative to the other ready operations.

However, you should never rely on queue semantics to ensure a specific execution order of operations, because changes in the readiness of an operation can change the resulting execution order. Interoperation dependencies provide an absolute execution order for operations, even if those operations are located in different operation queues. An operation object is not considered ready to execute until all of its dependent operations have finished executing.

Canceling Operations

Finishing its task doesn’t necessarily mean that the operation performed that task to completion; an operation can also be canceled. Canceling an operation object leaves the object in the queue but notifies the object that it should stop its task as quickly as possible. For currently executing operations, this means that the operation object’s work code must check the cancellation state, stop what it is doing, and mark itself as finished. For operations that are queued but not yet executing, the queue must still call the operation object’s start() method so that it can processes the cancellation event and mark itself as finished.

Note

Canceling an operation causes the operation to ignore any dependencies it may have. This behavior makes it possible for the queue to execute the operation’s start() method as soon as possible. The start() method, in turn, moves the operation to the finished state so that it can be removed from the queue.

KVO-Compliant Properties

The OperationQueue class is key-value coding (KVC) and key-value observing (KVO) compliant. You can observe these properties as desired to control other parts of your application. To observe the properties, use the following key paths:

Although you can attach observers to these properties, you shouldn’t use Cocoa bindings to bind them to elements of your application’s user interface. Code associated with your user interface typically must execute only in your app’s main thread. However, KVO notifications associated with an operation queue may occur in any thread.

Thread Safety

It is safe to use a single OperationQueue object from multiple threads without creating additional locks to synchronize access to that object.

Operation queues use the Dispatch framework to initiate the execution of their operations. As a result, operations are always executed on a separate thread, regardless of whether they are designated as synchronous or asynchronous.